Geocaching sounds like a lot of fun. It's a sport kind of like hide-and-seek, but much more complicated and, well, grown up. The WikiHow has a nice intro to this interesting "sport".

Geocaching is an increasingly popular, inclusive, fun and healthy pastime for individuals of all ages, as well as for groups like family, friends, classes, work teams, and youth groups. The core sport involves using a handheld GPS unit to guide you to a destination, where a hidden container ("cache") is stored. Once found, you log your visit in an included logbook, and optionally trade one of the many contained "goodies" for one of your own.

They go on to explain in some detail how it all works. Give it a read and have fun!